The Christmas Exchange
by NellyN
Summary: A few months after losing Amy Pond, the Doctor hears an unexpected knock at the door. It's Christmas Eve, and the Doctor has ended up in the one place that he can never, ever go. Surrounded by friends old and new, he must solve a mystery that could threaten the entire multiverse.
1. A knock at the door

A few months after the Doctor lost Amy, there came a little knock on the door. The Doctor looked up and frowned. He was not at all in the mood for society. He was sprawled on the floor of the TARDIS with a book in his hands and a mug of tea at his elbow, and as far as he was concerned, he could stay that way for the next ten thousand years.

"Hello," he muttered at the TARDIS. "Just send them off, will you? I'm busy."

The TARDIS yawped.

"Don't be like that, dear," He licked his lips and turned the page. "And I am busy. I'm… I'm…" He closed the book to look at the cover. _The Lonely Robot _by Amelia Williams. And he was holding it upside-down. "Coping. Grade A, number one, professional coping. Stop complaining. I'm making progress."

The TARDIS yawped again, a bit skeptically.

And there was another knock at the door, somewhat sharper this time.

"Fine," said the Doctor. "Very well." He got up and put his reading glasses in his pocket. "It's just to show that I can, though. They're not staying, whoever they are. I've had enough with houseguests. Who needs them? All they do is clutter up the place with their domestic and their bloody… mums. All we need are each other and the open sky. That's what I've always said. Haven't I always said that?" He rolled his eyes. "_Don't_ answer that. I don't want to talk about it. I really don't."

He stalked over to the door and threw it open. Then he had to lower his gaze by two and a half feet. A boy stood on his doorstep. A smallish, skinny lad of about seven. He had a red paper crown on his head. His expression was soft and earnest. A child who'd had an easy life, then. Never missed a meal. Doted on by loving parents, et cetera. Behind the boy, the Doctor could see a sliver of pale blue sky and damp, tan sand. That meant Earth. About 2014. No. Hold on. 2013. Or the mid-teens anyway. Which was pretty interesting, because the Doctor had last parked the TARDIS in the middle of a quiet nebula in the Third Great and Bountiful Human Empire, and that was a solid billion years off. Must've forgotten to set the handbrake.

He had been a little distracted lately, what with Amy going away and all the… coping. How embarrassing. He hadn't made an error like that since driving school.

"Hullo," said the boy.

The Doctor zoned back in. "Hullo."

"I'm Simon Smith." The boy extended a small hand.

He was very formal, so the Doctor had to respond in kind. They shook. "Nice to meet you. I'm the Doctor."

Simon cocked his head. "That's a funny coincidence."

"Is it?"

"My dad's a doctor."

"Right," said the Doctor. "Good."

"Look, I'm sorry to bother you," said Simon, crossing his arms, "but where did this box come from? Only we take this walk all the time and there's never been a box here before." He peeked around the Doctor. "It's nice."

"It's not _nice_," said the Doctor, too quickly.

Simon looked at his shoes. "I'm sorry."

"No." The Doctor grimaced. "I'm sorry. I've not been myself lately. Yes, it's funny inside."

"Bigger, I should think," said the boy. "Like mirrors face-to-face."

The Doctor smiled. "That's clever. Did you work that out yourself?" The Doctor evaluated Simon's hat. "Is it Christmas?"

"Christmas Eve, silly," said Simon. "May I come in?"

The Doctor leaned against the doorjamb. The kid _was _brave. Out for a walk on Christmas Eve, weird blue box, he just comes and knocks on the door. Bold lad. The Doctor liked him. More to the point, though: Simon had seen through the perception filter. Right through it, like it wasn't there. Which probably meant a loose wire in the perception condenser. Then again, it might mean something else.

The Doctor scratched his chin. "You'd better ask your mum."

"I'm not out with my mum."

"Your dad, then."

Simon grinned, showing off an impressive set of white teeth. "All right!" He dashed off, yelling _dad, dad_.

The Doctor was about to slam the door and zip off. He was thinking about visiting the Tin Vagrant. He could do with some spiritual counseling, and the Balance Gardens were a good place to be alone and have a lie-in. He started to dial in the coordinates. But something told him to go easy on the controls. This wasn't just loose wiring. And there was something else too. A flavor in the air. Like… it tasted like 2013, for sure, with its special tang of Twitter, wifi and climate change, but there was something off about it.

Weird.

He huddled inside the TARDIS for a moment, then muttered, "Oh, we might as well get on with it."

He took a deep breath, marched down the walkway and stepped outside.

It was a pleasant blue day, though chilly. The wind whipped, and distant clouds promised rain later, but the Doctor never bothered very much about weather.

He shaded his eyes and looked around. The TARDIS was perched a rocky coast. The sea foamed up through the rocks with every wave. Just beyond was a long, empty beach. A bit forbidding, but the Doctor ate forbidding for lunch. With vinegar. He took a turn around the TARDIS, trailing his hand along her side. He took a deep breath of the salt air. It had been a long time since he'd been out of the box. Not since Amy. The universe had taken care of its own affairs for months and months. _And we're well out of it, too_, _aren't we, dear,_ the Doctor thought, pressing his forehead against the side of the TARDIS for a moment. He was through. Utterly through. Retired. Finished. He didn't care if he spent the rest of eternity like a pearl inside its oyster.

That lad though. Sandy-haired kid. Skinny. Walked right up to the door.

A feeling like a frozen arrow shunted down the Doctor's spine.

He couldn't be here. That was both a physical fact—_he literally _couldn't_ be here, _as in, it was not possible—and an emotional one. If he spent another second here, his hearts would crawl up his throat and strangle him to death. He ran the rest of the way around the TARDIS. Then he hooked his foot on a rock and fell sprawling on the ground, knees scraped, head ringing. He ended up looking straight up Simon Smith's nostrils.

The boy stood over him. "Are you all right?"

"Unh," said the Doctor.

Another shadow loomed over the Doctor. A man held Simon's hand. The man was tall and thin and sandy-haired. He was about forty. He was trying to do stern, but his heart wasn't in it. He was mostly anxious but also very happy. Like he'd just run into a dear old friend, who he owed a great deal of money. The Doctor's own feelings drained down some inner crack, hopefully never to return—or at least, not until he got this lot sorted. Because this needed sorting in the worst way. This year tasted weird because it _was_ weird. This wasn't just the wrong planet, or the wrong year. This was the wrong_ universe_. And not just any wrong universe. The wrongest universe.

Rose's universe.

This was Bad Wolf Bay. And the man in front of him, wearing a funny green paper hat, was him. The other one. The metacrisis, half-human one. Let's call him John Smith—and why not? Smith was, no doubt, what he called himself. Rose, too, judging by the wedding ring. Rose Smith. Brilliant. Great. Brilliant. Not ghastly at all.

And their son. Their _son_.

"Well," said John Smith, settling his chin in his hand. "Isn't this wizard?"

#

"I don't—I would rather you didn't come in here," said the Doctor.

"Oh, would you?" John Smith pushed past the Doctor went into the TARDIS. He stood for a moment in the entrance and cocked his head. "Ah." Then he shook his head and strode up the walkway. "This is nice. I like it. The new design." He came over to the controls and brushed his fingertips over them. "Sim, don't touch."

Simon was just tall enough to look over the control panel. He looked like a kid who'd just been told that a present wasn't for him.

"The controls are so different," said John. "I thought I'd remember more. Can she hear me?"

The Doctor leaned against the door. He nodded.

"But it's not like it was with us," said John. He patted the TARDIS. "I'm sorry, old girl." His expression was wistful, but not miserable. "That's all right, though." He threw a grin over his shoulder. "You ought to see mine."

The Doctor stood up sharply. "You've got a TARDIS?"

"Don't be ridiculous," said John.

Simon bounced up and down on his trainers. "Can we go someplace?"

"No," said John and the Doctor together.

John settled down on one of the benches. Simon sat beside him, his feet dangling over the floor. John put a hand on the boy's clothespin shoulder. Simon was a pint-sized version of his father. They looked absolutely comfortable. The Doctor had to fight the feeling that he was evaporating.

"All right." John nodded. "Let's have it, then."

The Doctor came close to the control panel, feeling like he was sheltering under the limbs of a familiar tree. "Did you bring me here on purpose?"

"I didn't bring you here at all." John wagged his eyebrows. "Oooh, what now?"

The Doctor raised an eyebrow. "You come to Norway this time of year for the weather?"

"We come every Christmas," Simon declared.

"I don't think that's true." The Doctor came around the control panel and leaned against it. "Only the last two or three years, what? That's not bad timing. Considering your limitations."

"Yeah, I've still got it," said John, clicking his tongue. "It's trickier when you have to wait, though."

"Don't you get bored?" said the Doctor.

"Don't you get lonely?" John fired back. "I find ways to fill up the time, believe me."

"Me too. What am I doing here?"

"I don't know," said John. "But I bet it's something awful. Let's go find out. It'll be just like old times."

"I don't do old times anymore," said the Doctor.

"And yet here we all are," said John. He raised a finger. "Hold on."

John got up and walked to the front door. He opened the door and took a compact silver gun from the small of his back. The Doctor noted that he did it like it was a habit, smooth and quick. John opened up the weapon and tipped the bullets into his hand, dropping them into his coat pocket. He fired at the ground twice. Both times, the gun clicked empty. Then he took a handful of wet sand and worked it into all the mechanisms. The Doctor noticed that he'd done that before too. A familiar ritual. John threw the disabled weapon overhand into the sand. He slammed and locked the door. "Don't need that in here."

"That's littering," said the Doctor.

"Not in this universe," John replied. "They drag the beach once a week. Melt down the metal. They use it in the zeppelins. For the little parts. That was my idea."

The Doctor said, "Since when—"

"Not in front of my kid," said John. "All right?"

Simon looked up at both of them. Doing the math. _Different universe,_ thought the Doctor. Different rules. Very different rules. Too different. The Doctor nodded.

John said, "We're going to the Tyler mansion. You know how to find it. Try to get us there tonight. I've missed Christmas the last two years in a row. I'd like to break the habit. Go on, then."

"Can I pull the lever?" Simon pointed. "It's that one, isn't it? The big lever, there."

John and the Doctor exchanged a glance.

"Course you can," the Doctor told Simon. "What d'you think it's there for?"

Simon came over to the big lever. It was too tall for him; John lifted him up. "Now remember," John said, "it takes a bit of oomph. It's like the game we play at home with the SIDRAT. You can't just pull it. You have to commit." Simon didn't have any problem following instructions. The Doctor was barely able to get the coordinates in before the boy threw the lever with every ounce of his strength.

The TARDIS had never been to the Tyler mansion, but the Doctor had. Anyway, it had never been that difficult for the Doctor to find Rose. It was second nature. Literally.

#

The TARDIS flashed out of the vortex and back into realtime.

Simon's face was pale, but he grinned. "Blimey!"

John sighed. "Don't swear."

"But that was _amazing_." Simon jumped up and down, full of energy. "The SIDRAT is rubbish compared to that. She's really alive, isn't she, Doctor? Didn't you feel it, Dad?"

"Yes." John looked a bit shaken himself. He put a hand on the control panel to balance himself. "I felt it."

"I can't wait to tell Tony!"

"Go on, then." John cuffed him, gently. "You're getting underfoot anyway."

"I bet you could go _anywhere_ in this thing. Anywhere at all."

John set his jaw. "Yeah, it's like every-flavor beans."

Simon snickered. "You mean you could go someplace vomit-flavored?"

"I have done," said John. "Ask your mother."

The boy zipped out the front door. Then he came bounding back in. He saluted the Doctor. "See you later."

"I'm sure of it," The Doctor murmured.

John looked at him sharply, and the Doctor raised two fingers and shook his head one millimeter. _I won't take him_. There was no way on Earth or Heaven that the Doctor would have that boy in the TARDIS again. First, it would violate the Doctor's new rule against houseguests. Second, Simon was at least one-quarter Gallifreyan, and the last thing the universe needed was a tiny Time Lord banging around. The Doctor was bad enough on his own.

And finally, it would break John Smith's heart. That was obvious.

The boy ran back out again, calling, "Don't go again without me!"

He slammed the door behind him.

"Perceptive boy," the Doctor observed.

"Bane of our lives," John agreed. "Took apart the remote control when he was three. Now the only news channels we get are from Skellrax Nine—and they're two years old." John gave a stage-sigh. "Still, I suppose we have to keep him now." He paused. "You know it's funny, I was just thinking. They'd be taking him for initiation about now. Back home. Can you imagine?"

The Doctor shook his head. "No."

The Doctor's initiation was still fresh in his mind, though it had happened more than a thousand years ago. You never forgot the Untempered Schism. It broke everyone. It was always here, always now, always just an unguarded thought away. It whirled away in your mind for the rest of your life. A Gallifreyan child would never become a Time Lord without looking into the Schism, letting it eat at the sensible parts of his soul. But there was still a part of the Doctor that was hurt, and regretted, and wondered: _what if I hadn't_?

Simon was safe from all that. Thank God.

John gripped his temples. "Look, if this is going to work, we have to get one thing clear. Right now. I didn't want to talk about it in front of Simon because he wouldn't understand. I don't want him to. Ever. But here it is." He took a deep breath. "I'm not _him._ And I'm not you. I'm not a bit the Doctor. I've learned a lot since the last time we met. Things you can't know. Human life…" He shook his head. "It's bigger on the inside. And my choices were—are—human choices. And that's all you have any right to expect from me. All right?" He planted his feet and lifted his chin.

Silence settled between them like an unloaded gun.

"Fine," said the Doctor.

"Good." John leaned forward and lowered his voice. He smiled. "Would you like to see the SIDRAT now?"

"Depends what it is."

John angled his head toward the door. "Don't worry. You're going to love it." John held the TARDIS door open. "Step into my parlor."

The Doctor stepped through, and John followed him. They were in a large cellar. Like a warehouse.

In the center of the warehouse was a circle of mirrors. There were twelve of them. Ten feet tall, placed at intervals. Like a clock. Or Stonehenge. They were all wired into each other with thick cables. And in the center of the circle was the TARDIS.

The ring of mirrors felt like a time-engine, or a paradox machine, but was neither. The Doctor didn't have a name for what it was. SIDRAT, then. Backwards TARDIS. The system was well-balanced and ran clean, with a solid thirty-second buffer. And it was _dead on_ accurate. To the millisecond. The Doctor could feel it. The SIDRAT was twice as accurate as the TARDIS on her best day. Of course it was only about a tenth as powerful, but what it lacked in juice, it made up for in precision.

"Gorgeous." The Doctor, turned on the spot, his mouth slightly open. "Unbelievable."

John grinned. "None of your shoddy workmanship, either."

"Hey!"

"Don't get tetchy." John rolled his eyes. "I don't have build everything out chicken wire and popsicle sticks anymore. We've been working on it for almost a decade. You wouldn't believe the money. You're looking at a quarter of the Tyler inheritance here. Oh, you should hear Jackie talk."

The Doctor kept his hands behind his back as he walked the inner perimeter. He was trying not to disturb the balance; with the TARDIS here, there was going to be a lot of energy buildup. Enough to fuel a big jump—or a big bang. Best to be careful. "What's it for?"

John shrugged. "Rose and Mickey worked out the basics before I got here. Then I tuned it up and put it into alignment. Human technology with that little bit of Time Lord. It's good, isn't it?" John flicked one of the mirrors. It chimed like fine crystal. The Doctor felt the resonance pattern and shivered. John continued, "It's not really meant for travel. Can't send a person very far, and can't bring them back. We're mostly using it to repair the Wall."

"The Wall." The Doctor looked at himself in the mirrors and bit his lip.

"The boundary between your universe and ours," John explained. "You remember all the hopping we used to do. Canary Wharf. The big displacement. It left a lot of micro-tears in reality. Most of them open onto the Void, some to your universe, some to… other places. Of course we repaired the really big ones a long time ago. Now there aren't many left much bigger than a cat door. Some are small as a flea."

"Knitting the universe back up. That's delicate work."

"It keeps me out of trouble."

The Doctor scoffed. "Really?"

John smiled coolly.

The Doctor folded his arms. "It doesn't. Keep you out of trouble. Does it? Because things are still getting in. Or maybe falling in. And I don't mean me. Things that would make a family man carry a gun when he's out with his his seven-year-old son."

"Oh, that's been happening for ages." John waved the idea away. "Since the first time we came here. There's a hundred-pound bounty out if you bring the body of a Dark Thing to the Ministry of Dimensional Integrity. A hundred and twenty if it has teeth or claws. Don't look at me like that. We don't _participate_. When we catch them, we bung them back through the Wall. Simon can hit one with a tranquilizer dart from twenty paces. I doubt that's it."

"Then what's the problem?" The Doctor wondered. "Why am I here? Why on Christmas?"

It was a rhetorical question. The boundaries between universes, between timestreams, were thin on Christmas. All that psychic energy directed at one day was bound to loosen things up. It was one of the reasons the Doctor did Christmas all the time and never did, say, Palm Sunday. And this universe was already vulnerable. A lot more vulnerable than the Doctor had planned for. Canary in the coal mine, this universe. Always had been. What happened here today could happen everywhere tomorrow.

"Feel free to let me know when you've figured it out," said John.

A familiar voice echoed down the stairwell. "John?" Both men froze, for different reasons.

"Oh, here we go," said John, between his teeth. He looked to the heavens.

The Doctor pivoted and headed back to the TARDIS.

John caught his elbow. "Don't you dare."

Rose came down the cellar stairs, talking all the while. "I didn't expect you back so early. Look, a check came for you from UNIT. Christmas bonus, I think. I know you don't want a salary, but they keep insisting. We can send it to Oxfam if you want. I don't care. And the wards keep going off. I don't think it's an incursion, I think it's just this snow. But you might want to check the traplines."

John kept a firm grip on the Doctor's elbow. "Sure. Send it to Oxfam. I don't care either."

"And you'll do the traplines? Tonight?"

John didn't respond.

Rose said, "It's a mess down here. Where are you, anyway?"

"The SIDRAT," called John. "Inside the matrix."

"That old thing. What're you doing in there? You know it's dangerous."

The Doctor arched an eyebrow. _Dangerous?_

John grimaced and shook his head. _Not a big deal_. "I think you should come in here."

She softened her tone. "You all right?"

The Doctor was not all right. He thought he might pass out.

John tapped his foot. "In your own time, Rose."

"All _right_. Hold your horses." She inched between two of the mirrors. She held a thick manila envelope in one hand. "Look, I didn't want to spend Christmas Eve alone, so I invited a few—oh. Who's—oh."

She'd seen the TARDIS. She went stock-still, and all the blood drained from her face. She tried to speak and failed.

"Now, Rose," said John, trying to calm her.

She turned to him, her eyes blazing.

John raised his hands in a gesture of surrender. "Just listen. I didn't call it here. I wouldn't have the first clue. You know that. I don't even want it here. Not in our house. And you know that." He paused. "And it's Christmas."

"Yes." She had to force the word out. She cleared her throat. "Yes, of course. Right. Christmas." She seemed to catch up to the moment. She put herself together and looked straight through the Doctor. "I suppose they have been a bit quiet lately, our Christmases. Why don't you come up, Doctor? We're having a little get-together. You'll get a real kick out of the guest list." She glanced at John. "All our friends are coming."

"Oh, _perfect_," John burst out.

"Oy, don't blame me. You didn't tell me you were coming home! And you didn't tell me you were bringing him." She kicked at one of the mirrors, throwing it out of alignment. The whole system twanged. There was even a sympathy moan from the TARDIS.

Rose took a breath. She spread her hands. "It's fine. It's fine. I'm sorry."

"I'm sorry," said John. "You're right. I should have warned you."

"Yes. But I'm fine." She ran into his arms. "I'm glad you came back." John hugged her. He looked like the cat who'd got the cream.

The Doctor stood to the side, feeling about as alone as he'd ever felt in his whole life. Letting John go, Rose reached out and squeezed the Doctor's hand. It startled him quite badly. Rose noticed. Well, she would. He'd just about jumped out of his skin.

"Let's start again," Rose said. "I'm glad to see you, too. 'Course I am. But I wish you weren't here. Because your being here means we're all in danger."

The Doctor nodded. "I think that's probably true."

Rose drew herself up. "But if we are in danger, then it's good. I don't know how it's possible, but it's great that you came. And we shouldn't be afraid of each other. We don't have time for it." She tossed her head. "Come on up. I'll show you the house. And I won't surprise you. It's a small party but you know everyone who's coming. Sort of."

The Doctor bit his lip. "What does that mean, 'sort of'?

She ticked the guests off on her fingers. "The heads of UNIT and Torchwood. And the President, of course."

"Right." The Doctor nodded. "And who're they, when they're at home?"

"Well," said Rose. "Respectively, that's this reality's Martha Jones."

John piped up, "And our Donna Noble."

"And our Harriet Jones," said Rose.

"You see," said John. "As soon as we settled in we got the band back together."

"We had to," said Rose. "Right away. The Wall is so vulnerable. And this reality hasn't got a Doctor."

At that point the Doctor turned around, got into the TARDIS, zipped off to the other end of the Universe, and started a new life as a turnip farmer on the Lost Moon of Poosh. He was a lot of things, but he wasn't a fool.

Oh, who was he kidding?

Of course he was.

And who didn't want this, anyway? Christmas with the family.


	2. A big incursion

But perhaps not quite so _close_ to the family.

Sit across the gravy boat from Donna Noble on Christmas Eve? That would be a kind old universe. He cleared his throat. "Can't make it, I'm afraid."

"Do you think we're going to bite?" said Rose. She was smiling now, but she was also holding John's hand, her grip tight as an anchor chain.

"It's not that." The Doctor hooked a thumb at the TARDIS. "She doesn't like your SIDRAT. Unless you want dinosaurs to come through the Wall, I need to move her before she starts phase-shifting." He coughed. "Been doing a lot of walls lately." He inched backwards, feeling for the door. "I may make it my back-up career. I mean my back-up back-up career. Ta."

He pushed open the TARDIS door, slipped inside, and shut it behind him.

The TARDIS was itchy. Running a bit hot, too.

"I'm sorry, love," the Doctor said. "My fault. How about a quick dip in the Atlantic? Alternate universe. Maybe it fizzes."

She twanged.

"Well, how do you think I feel?" He put his hands on his hips. "If it was anyone else…"

The TARDIS sighed.

So did the Doctor. "The SIDRAT is not _prettier_. Size isn't everything. Come on." He dialed in some coordinates and took off.

#

Rose and John Smith watched as the TARDIS faded away.

Rose kept her eye on the spot where the TARDIS had been. "I suppose he's coming back."

_I don't think he went anywhere,_ thought John. It was going to be a long Christmas. He rested his chin on Rose's shoulder. "You all right?"

"Yeah. You?"

What a question. "I'm always all right."

Rose bit her lip. "Could dinosaurs really come through the Wall?"

John shrugged.

"You always were such a liar," said Rose.

John nodded. "Nothing changes."

#

The Doctor did not go to the ocean. He had his fist around the end of the thread; he had to follow it now. If it had been anyone else, anything but this reality, then maybe he could have let it go. But he couldn't let anything happen to them. Rose, John and Simon: that was the only happily ever after the Doctor ever expected to get.

So. One last job. Like in the films. One last job and then retirement.

He was aiming for the roof, but he got the nursery. Though the children were not so little anymore, so maybe they called it something different. Crèche. Egg. Kindergarten. No, wait—he _was_ getting old—not egg. That was for chickens. They probably called it the game room or something. Anyway, he landed there, scattering Lego and footballs.

Two boys turned around from their video game to stare. One was Simon and one was Jackie's boy, Tony.

"I told you!" said Simon, pointing. "I told you!"

"Sorry," said the Doctor. "Health and safety department. Toys section. All looking fine here." He tapped his forelock. "Merry Christmas."

He shut the door and set sail for the roof.

Next he found himself in the big marble foyer, nestled between the grand staircase and the Christmas tree. That was the problem with people who had money. The houses were bigger on the inside. And the outside. It was like playing "Pin the Tail" on a donkey that was fifty feet tall. For goodness sake. He shut the door again, then furrowed his brow and stepped back into the room.

No, he wasn't seeing things. The decorations on the tree were moving.

The Doctor approached cautiously, fist clenched around the sonic screwdriver. Then he chuckled. The decorations were all clockwork. Silver doves and cardinals flitted from branch to branch. The globes were real globes, too. Each one was a tiny model of one of the planets or space colonies the Doctor had visited. They rotated around the tree on their real orbits. There was a little Raxicoricofallipitorious. Several Earths. Even a Gallifrey.

_Delicate work_. It had been good for him, then, this house. At the moment he had neither the patience nor the steady hand to make things like this. He heard voices approaching. Rose and John. He slipped one of the clockwork doves off the tree and tucked it into his pocket. He got back into the TARDIS.

"Here we are, that's better," he said, a moment later.

He stepped outside. The Tylers' mansion had a broad, flat roof. A cold wind whipped in from the north, carrying snow and ice. But from here you could look out at all of Alternate London. It looked in fine fettle too. No doubt it carried its scars from the last pandimensional fete. But the lights were still on. And the stars were all still there. And the world was still turning.

Round and round and round and…

"Ha!" he said. He dropped onto his hands and knees and crawled to the edge of the roof. Around the edges of the Tyler-Smith property were lanterns. They weren't Christmas lights. They were gas-fired torches, set at intervals of about twenty feet. Rose's trapline. "The wards keep going off."

God, but he was getting slow. The wards should have been the first thing on his list.

He leapt back into the TARDIS, whirled around the house, and brought her down right next to one of the torches. They were fine old contraptions. A bit weird. The structure was cast iron, with a kind of iron lunchbox at the base, half-buried in snow. A post about three feet tall was welded onto it. And on top of the post was a real London gas lamp. As in, from the 1800s. A scavenging job if there ever was one. The Doctor pressed his stethoscope to the lunchbox, then his ear. Nothing. Empty.

He looked up, brain going a mile a minute. He wiggled the iron post. Then he hooked a foot around it and tipped it over. It made a sound like _twang_. The post hit the snow and the pilot light flickered out. The movement pulled a thin black wire out of the ground. It attached underneath the lunchbox and ran… ah. Into the house? Into the cellar, maybe? He flicked his finger over the wire. _Twang_.

"Oh, Rose," he said. "You _have_ been busy."

"Mickey's idea, actually." Rose stepped out of the shadows. Her winter coat was TARDIS blue. "Sorry." She tucked a strand of blonde hair behind her ear. "Am I interrupting? Cause I could go and come back later."

"I wasn't talking to myself," the Doctor said.

Rose grinned. "Could have fooled me, mate."

Silence. The Doctor cleared his throat. "So. These traplines of yours. How do you know when you've caught something?"

"The light goes out." Rose righted the one he'd tipped over. She kicked the box, then lit the lantern with a Zippo. She made sure it was steady, then kicked snow back over the wire. "I'll show you." She beckoned and trod away.

The Doctor hesitated, then followed her.

Rose said, "They won't keep out anything really big. Or really small. It's more like an alarm system. It tells us when things are coming through the Wall."

"What do you do when it goes off?"

"We have a shelter."

"So it's like the Blitz." The Doctor kicked a pile of snow. "You go underground."

"We live," she said. "We wait for the storm to pass over, and then we clean up, and in between, we have our lives and it's nice and hold on a minute, are we having this conversation?" She glared over her shoulder.

"That's not what I meant."

"There are lots of ways to fight. Not everyone can do it like you."

"I know."

She crossed her arms and looked away from him, up at the stars. "You're different. It's not just the face."

"It's been a lot longer for me."

"How much longer?"

"Do you want it in number of years?" he said. "Or number of wars?"

"Not that different then." A smile played across her lips. "Maybe it's me."

The conversation had taken them around the south wing of the mansion and into a snow-covered garden. A small frozen lake was an ideal spot for ice-skating, and the igloo was just large enough for two boys to play in. At the far end of the lake stood a small copse of pine trees. And between the lake and the trees stood another one of Mickey's Victorian lanterns. This one was unlit.

Rose beckoned. "Half the time it's a false alarm."

They crept up to the box. Rose took a huge hammer from her coat pocket. Ignoring the Doctor's raised eyebrow, she tapped the side of the box.

From inside, a skittering.

"Ho-ho." Despite himself, the Doctor grinned. There was something in that box that nobody had ever seen before. "Bad luck, eh?"

"Get ready," said Rose. "I'm going to let it out."

She shifted her grip on the hammer. Then she carefully tipped the trap backwards and pulled a catch on the bottom. A small door sprung open. Rose dropped the trap and stepped away. She hooked her finger around the Doctor's sleeve and pulled him back with her.

Nothing came out. The box was black and silent inside.

The Doctor put on his glasses and squatted low to the ground. "Come on," he coaxed it. "It's my last time out. Thrill me."

An animal slid through the trap's small door. It was gold. Not golden. Actual, living gold, such as you did not see in this dimension. It was long and thin and writhing, with a broad, tufted head, small feet, and glittering red eyes. The mind said _lizard_, but that was only because it was the nearest word, not the right word. Under its feet, the snow evaporated, shrouding it in mist and steam.

"Beautiful," said the Doctor. "Hello."

Rose closed her hand on his shoulder.

He looked up at her. "And you call _that _a Dark Thing? Shame on you, Rose Tyler. You've been in this dimension too long."

"It's not just them," said Rose. "And I didn't think of the name."

At the heels of the first creature came a second, identical in size and shape.

The Doctor murmured, "Who thought of the name?"

Rose just looked at him.

A third creature came out of the trap.

"Exactly how big _are_ these boxes of yours, Rose?" the Doctor asked.

"I think we should go inside the house."

"It's all right," said the Doctor. "I think they're afraid."

The creatures huddled together, twisting around each other. Three pairs of eyes peered at Rose and the Doctor. They gave nothing and took nothing.

"It's not just them," Rose repeated. "This is a big incursion."

The Doctor chuffed. "_I'm _a big incursion." He thought about that for a moment. "And we're going to pretend I didn't say that."

A fourth and fifth creature emerged. They joined their friends. It was turning into a knot. Or a conglomeration.

"Doctor, listen to me. The wards." Rose swallowed. "They're out all over the property. They've been going out all day."

"All right." He stood. "Let's shift, then." He looked into the glittering eyes. "But maybe… slowly. Do you hear me, Rose? Go ahead of me. And don't run."


	3. Bad news

The Doctor slipped his hand into his pocket and closed his fingers around the sonic screwdriver. He kept his eye on the heaving knot of gold beasts as he called to Rose over his shoulder. "Are you at the door?"

"Aren't you coming?"

"I'm right behind," said the Doctor. "Let me know when you're there."

The goldies didn't approach. The Doctor quietly tuned the sonic to deliver a gust of gentle heat, just a few degrees warmer than a hairdryer. Harmless, but it would startle the hell out of most animals and some people. "Rose?"

"I'm there."

He fumbled a bit; his fingers were getting chilled in this weather. "Have you opened the door?"

Pause. "Yeah, it's wide open. But be quick, all right? I'm going to bar it behind you."

"Step inside," said the Doctor.

"What?"

He put an edge of steel into his voice. "Don't argue with me, Rose. Just get over the threshold."

"Fine, fine, I'm over. Now what?"

"Shut the door and lock it!" He pulled out the sonic screwdriver and ran onto the frozen pond, heading east. His feet slipped but he managed to propel himself across the pond with only a minor loss of dignity.

Rose gawked. "What are you _doing_?"

"Don't worry about me!" The Doctor ended up ankle-deep in a snowdrift. "Have to go get the TARDIS."

She spread her hands in exasperation. "It's the other way!"

The doctor grinned and waved, not stopping.

"Doctor, anything could be out there! And when I say _anything_—"

Rose's scolding faded into the distance. The Doctor had spotted another trap with its pilot light out. He dropped to the ground and slid across the snow. He ran out of momentum right in front of the box. Shivering and half-soaked in melting snow, he flicked the box with his fingertip. There was no animal skittering from inside, but there was something in there. Empty boxes echoed. They didn't go _slosh._

"All right." Without bothering to get up off the ground, he tipped the box backwards, so the little door was facing up. "Let's see what else you've brought the Doctor for Christmas."

He sprung the latch and the door popped open. He came close to leaning over the box, then thought better of it. A wise choice. What happened next was like one of those trick cans of nuts with had a snake inside, only a lot weirder and scarier. Thick black strings sprung out of the door, dripping with some kind of clear, plasmic goo. He would have gotten smacked right in the face. And that would have been bad, because this thing was folding a bit of reality around itself. Just looking at it hurt.

He blinked and looked away. "Oh, you're very far from home, aren't you?" This thing wasn't just from the next dimension over. "Love what you're doing there, I really do." He really did. "Quantum blurring? Brilliant. I've never seen a complex lifeform do that before. But there's a time and a place, eh? I think… yeah. There you are."

He gave it a gentle nudge with the sonic's heat setting. The creature withdrew into the box. The Doctor closed the door and tightened it with the screwdriver. Best to keep that where it was for now.

The third and last trap the Doctor opened contained a pair of brown field mice. They chittered at him and cowered in the corner of the box. At first he thought they were just mice, but then he soniced them, tuning the screwdriver up to the resonant frequency of rift energy. Their fur became suffused with a pale blue glow. They'd been on quite a trip themselves, passing through the Doctor's home dimension—and picking up a little bit of Void energy along the way. The Doctor supposed he would have to take them home, but when he reached into the box, one of them bit his finger. He dropped the box and they both dashed out across the snow. The Doctor sucked on his bleeding finger, thinking _well, if this dimension didn't have glowing mice before, it does now._

And there was an inkling of what life must be like for Rose and John on the edge of the Wall. He'd imagined the two of them would be bored to tears out here, but really, it wasn't that much different. The Doctor reset the trap, lighting the lamp on top with the sonic. He laid on his back in the snow, shivering, looking up at the stars and letting the spool out on his thoughts. It was important to move—and soon. Time was ticking away. But he didn't want to. His understanding of the problem was developing. He was getting the picture. And it was not a particularly good one.

At least, not for him.

The Doctor had a healthy respect for survival, but he wasn't afraid of death. He had already lived a lot longer than anyone had any right to expect. He would certainly die someday, as all things did, but he assumed he would be far too busy to notice. After the three hundredth epic battle and the nine hundredth impossible choice, mortality just didn't hit you the same way.

However, that still left a fairly long list of things he never cared to meet. There was plenty worse than death waiting for him out there. If he was in fighting form, perhaps he would have met it with a laugh. That was what it meant to be the Doctor, after all. But his recent losses were a millstone around his neck. All the youth, beauty, humour, courage, kindness and distraction his friends brought onto the TARDIS was gone. He saw nothing in his past and nothing in his future but these good qualities washing in and out of his life like tides. The idea made him tired.

But carry on, carry on… no one had ever been promised a reward, except in fairy-stories.

#

He trudged the rest of the way around the mansion, keeping his distance from the rest of Rose's traps. A sense of relief and homecoming enveloped him as he snapped his fingers and stepped through the door of the TARDIS. But he couldn't hide in here forever, either. He tapped a few buttons and tugged on the zigzag plotter. The ship shifted under his feet. It was just a short hop—hardly any time at all, and none of it in the vortex—but the Doctor let her cycle out for a few minutes. He left the console and went into his workshop. He began digging up different things he might need. Fermer couplings, yes, and a couple of spools of Number Seven cable. He threw them out the door and into the control room. Oh, hello, there was his old chronoway—always useful when you were getting fancy with reality, but to really push its limits you had to build it into a sugar circuit and for that you needed—ah, there, a cooling fan. And a sub-atomic rederecompressor. And sugar, of course. For the caloric energy. The Doctor liked to keep his engineering calories in the form of chocolate bars. They had quantum stability and the added benefit of being a nice snack later. He chucked a box of them onto the pile.

By the time the TARDIS cycled down, the Doctor had wires and equipment spread across the control room floor. He'd pulled the insulation casing off the time rotor and attached the cable with magnetic clips. Without looking behind, he took a loop of cable over his arm and spooled it out as he backed out of the door. He was pleasantly surprised to find himself back in the cellar, snuggling up to the SIDRAT but not quite inside the matrix.

"Oh, getting our bearings now are we?" he muttered at the TARDIS. He held a bag of thumb-sized resistors between his teeth. "Good girl." He put the resistors on a passing worktable, got down on his hands and knees, and began stripping the insulation from the cables that tied the SIDRAT to the forest of servers that powered it. John Smith had given his project a big brain, no doubt about that, but brains were cheap. The Doctor and the TARDIS were about to give the SIDRAT a soul.

He felt eyes on him shook his head. "If you're going to hang about you could help me."

When they didn't answer, the Doctor glanced over his shoulder. The cellar stairs were behind him, steep and made of rough wood. Rose Tyler sat at the bottom of the stairwell, her fingers folded around a Christmas mug. John stood, leaning on the bannister. They'd clearly been waiting for a while. He returned to the task.

"It's been hours," said Rose. "Where have you been?"

Hours? "Oh, you know me. Work, work, work. It never stops." The Doctor melted some solder with the sonic and made a bond between the TARDIS and the SIDRAT.

"Your fingers are blue," said Rose, her tone casual.

"Are they? Huh."

"You can have tea," John offered. "We've just made a fresh pot."

Rose said, "Second one."

"You're very kind. Ow." The Doctor had electrocuted himself. He stuck his stinging thumb in his mouth. "Put it in a travel mug, eh?"

Rose broke the silence. "Where are you going?"

"Nowhere at all." The Doctor put the screwdriver down and turned around, sitting lotus-style on the cellar floor and looking up at them. He had already been hundreds of years old when Rose was born, and at this point he had something like two hundred years and a full regeneration cycle on 'John Smith,' but right now they made him feel young. Mum and Dad waiting up after curfew.

The Doctor took a deep breath. "I'm going to blow a big hole in reality. In your basement. Sorry." He paused. "But not really. Cause I think I'll probably be saving the multiverse."

Rose looked at her shoes. "How's that?"

The Doctor sighed. "Oh, do I have to explain it all myself?" He spread his hands. "You know. 'John' certainly knows. I can see it in his face."

John's expression was cold, and he wouldn't meet the Doctor's eyes. He might have settled in to this funny little life, the Tyler-Smiths raising a tender human family on the edge of the Wall. He might even be able to convince himself, most days, that he was human like they were human. But it was hard to hold onto those big illusions when you were looking at yourself in a mirror. John Smith hadn't wandered Bad Wolf Bay every Christmas because he was nostalgic, and he didn't carry a deadly and indiscriminate weapon to protect himself from goldies and Dark Things.

"You shouldn't call it the Wall," said the Doctor. "It's more like a veil, isn't it? Or a sheet or a screen. Thin and porous and constantly moving. I thought about it while I was up there on your roof. And when I was laying in the snow looking up at your stars. They're the same ones."

Rose looked at him blankly, but the Doctor wasn't talking to Rose. He was talking to John. "You remember that war we fought in all those years ago. Planets in the sky."

John gave him a deadpan look; that battle was the moment his entire life had pivoted on.

The Doctor snapped his fingers. "We fought that war in _my_ dimension, yeah? But we saved your stars too. We saved everyone's stars. So the Wall can't be what we think it is. It's a lot thinner than that."

John nodded.

"John," said Rose. "What's he on about?"

John didn't reply.

"Look," said the Doctor, "I know I'm just the parlour-maid and it's not my place to speak, but you have to tell her."

John steepled his fingers. He hesitated, not because he was afraid to say it (though he was), but because he was trying to fit it into words. Gallifreyan had loads of words for these kinds of problems, but twenty-first century British English was fairly limited in the terms it had to describe quantum entanglements. They had barely discovered bosons, after all, the lambs.

John said, "You know in films how, when there's a forest fire, all the little animals run out ahead? The squirrels and birds and things, they dash? All together."

Rose glanced at the Doctor. "Uh-huh."

John made a spooling gesture.

Rose's brow furrowed. "You're saying there's a 'fire' in some other dimension and its… what… pushing things through the Wall? Causing incursions?"

"Eh." John grimaced at this imprecise explanation.

"He's saying," said the Doctor, "when you find a whole lot of small animals where they're not supposed to be, you should wonder if they're running away, and then you should wonder if there's something catching them up."

"Catching them up." Rose's mouth was a hard line. "Like what?"

"Well it would have to be big," said John.

The Doctor added, "And it's affecting more than one dimension. You're getting refugees from _way_ out of town. Including me. I bet you've been getting them for a while, right?"

"Incursions?" She shrugged. "All the time."

"But more now," said the Doctor.

Rose nodded. "The last few years."

"And stranger things are coming through. Bigger. More dangerous. Dangerous enough to scare you."

"Dangerous enough," said John, "to scare _me_."

"And now it's Christmas," said the Doctor. "Woof."

John sat on the cellar stairs beside Rose's feet. She scooted over to make room for him, and then he rested a hand on top of her shoe, not even thinking about it. Just because he was worried and thinking.

The Doctor smiled stiffly and looked at his fingernails.

"So this terrible thing," said Rose. "It's in the Void, right? Got to be. Because the Void is everywhere. It's the only space that touches every universe. The Void… is… the container that the multiverse is in."

"That's not even remotely what's going on," said the Doctor. "But yes, that's what's going on. And whatever this void creature it is, it's going to keep chasing or biting or burning or growing. And it's going to push bigger and bigger things into your universe and mine until…"

"Until what?" Rose looked at him with perfect trust.

"Until whatever," said the Doctor. "The details can't possibly make a difference."

"What's going to happen to us?" Rose insisted. He saw Simon in her face.

"Well," said the Doctor. "Very soon, you'll be overrun with large, fast and bitey creatures from other universes. It'll make the worst incursion you've ever had look like a Sunday teatime."

Rose shivered. "All right."

The Doctor wasn't finished. "And then the Wall will crumble. All the Walls, everywhere. It'll take a while, but not that long. A few years. Less. And after that… it's impossible to say. Maybe everything will turn into the Void. Or maybe it will just be that: one big universe with every possibility fulfilled, all at once." The Doctor grimaced. "To be honest, I don't fancy my chances. There are versions of myself that I'd _really_ prefer not to meet."

Rose closed her eyes and set her jaw. She put her mug down and closed her fists in her lap. Beside her, John waited with a partner's patience, perfectly calm. Rose was a very much a woman of her time, and one of the most human beings the Doctor had ever met. But she'd also had the time vortex in her head, just like a Time Lord. Her advice had always been worth waiting for. She said, "What do you need us to do?"

"Bless you, Rose." The Doctor leapt to his feet. "Is the Christmas party still here?"

She opened her eyes. "I sent them away. With everything that was going on…"

"Well, you'd better get them back," said the Doctor. "Big night. Lots of Things coming though the Wall. You're going to need people to take care of business." He lifted a finger. "Hold on—I've been meaning to ask you, cause you were going pretty fast and it went by me the first time—you put _Donna_ in charge of Torchwood?"

He got a pair of frosty stares.

John arched an eyebrow. "You got a problem with that?"

"Okay." The Doctor raised his hands and changed the subject. "Do what you do. Defend this house. Defend London. Defend the world. Be wonderful." He pivoted back to his work, the sprawl of SIDRAT and TARDIS parts. "Meanwhile, I'm going to do what _I_ do, which is go someplace nobody's ever been before and meet something that nobody's ever met and cause a terrible ruckus and possibly save the world. We've got something like—" He checked his watch— "four hours til Christmas Day, so let's make it happen."

Despite herself, Rose smiled. "Yeah. All right."

The Doctor grinned back. "That's the spirit."

John hadn't moved. His expression was dark and grim. His face said something the Doctor didn't want Rose to figure out.

_Don't tell her,_ thought the Doctor.

John's expression gave away nothing: _no promises_.

That was the trouble with mirrors and illusions. They went both ways.

#

At the top of the stairs, Simon Smith had the stethoscope from his doctor kit pressed to the cellar door. He was wearing red flannel pyjamas, slippers and a dressing gown, but he was still a bit cold; with all the stone and marble, the manse got drafty on winter nights.

Tony tugged on his sleeve. "What're they _saying_?"

Simon put his finger to his lips.

Tony was nine, and the two boys were best mates, even though they were technically uncle and nephew, which Simon thought was funny. With their families involved in Whitehall politics, neither boy could go to ordinary school. They'd been educated in the halls of Torchwood and UNIT, by the books in the Tyler library, by their parents, and by each other.

By necessity, it had been a good but narrow education. At an age when most children had barely mastered the ABC, Simon could win a quiz night on quantum physics. He _murdered_ maths. He could hold his own in adult conversation about politics or the Wall, favourite topics among family and friends at the manse. He consumed books at an astonishing rate. He knew all kinds of things about other planets and galaxies and alien cultures, even though there was no way to visit them from Earth; he could name every clockwork planet on the Christmas tree, and tell a story about it. But there were great huge gaps in his knowledge, where dull things like geography and astronomy and biology were supposed to be, and he preferred to keep it that way. Anyone who managed to corner Simon for some traditional education encountered a student who was perfectly polite, but impossible to reach.

Tony was the same, though he had different reasons. Simon knew that Tony was clever enough, but _clever enough_ wasn't clever enough for this family. No wonder Tony preferred to spend his days playing ball or darts or videogames—or trapping Dark Things with Simon in the halls of the manse. He was good with his hands, fast and strong, and Simon worshipped him as little brothers do big brothers. Tony was a kind boy, neither jealous nor manipulative, and most of the time he seemed content to be left at the edges of the debates and decisions that centred on this house.

Still, there was this sitting in both of their futures: eventually—a long time from now, but _eventually_—Tony would inherit the manse. Simon thought about it all the time. Simon knew all about the Ministry of Dimensional Integrity, with its fancy headquarters on top of the Gherkin. But the manse had _real_ power.

The conclusion Simon drew from all of this was that being Tony Tyler's best mate was a serious responsibility and that it must be taken seriously. Simon had to think about all of the things Tony didn't think about, and learn all of the things Tony didn't care about. It was loyalty, not curiosity, that motivated Simon's eavesdropping.

Well. Maybe a little curiosity.

What Simon overheard frightened him. He knew that he had just learned a frightful, adult secret. He wanted nothing more than to talk to his mum about it. But she mustn't find out that he knew. Dad either—he never talked about the Wall or Dark Things with Simon. And Tony _definitely_ couldn't know. If Simon could even convince Tony to believe it, it would just scare him. So. He would have to face it on his own. Not for the first time.

"Sim!" Tony whispered, pinching Simon's arm. "I'm getting bored. What's going on?"

"Nothing," said Simon. "They're just talking about politics."

Tony did a mock-yawn. He politics insufferable. So did Simon. But Tony didn't like politics because he didn't understand it. Simon didn't like it because everybody lied. Even his mum. Simon wasn't allowed to lie, but he still did sometimes.

Tony said, "Let's do something else."

"Okay." Simon folded up the toy stethoscope and put in his pocket. "Let's play hide-and-seek."

"No way. You always cheat."

This was true. "I promise to stay inside the manse." Simon crossed his heart. "See? Honest. And I'll give you a quid if you find me. But the big fireplace is base. And you have to count to three hundred."

Tony examined this deal for flaws. "Five quid and you can't run."

"Sold." They shook hands. Tony ran for the living room.

Simon waited until he heard Tony counting. Then he ran for his life.

The manse had an old dumbwaiter that had once been used to haul food and laundry around. The boys weren't supposed to play in it, but Simon did.

He didn't let himself feel anything until he was inside. Then, he almost cried, his small hands shaking. His beloved manse, the safest place in the world, surrounded by terrible monsters. _Overrun_, the Doctor had said. That was a new word for Simon but he understood it and he didn't like it. And it wasn't just the house that would be overrun. It was everything. Everything he cared about.

And everything anyone cared about.

He caught his breath. _Buck up, Sim_, he thought at himself. That's what his dad said when Simon got too upset. _Buck up. When things get bad, that's when you need to have hope. _Simon knew he had to be brave. His ears and heart were both burning. All of his education, all of his instincts, and all of his upbringing told him that he needed to be inside that box the next time it took off. Simon was very good at hiding. He could manage that no matter how frightened he was. That was another thing his parents taught him: _when things get bad, we're the good guys. We're the ones that help._

If what the Doctor said was really true, then there wasn't any time to waste.

_#_

The thing that John Smith couldn't say, his wife didn't know, and his curious little son didn't overhear:

The TARDIS clung to reality. She was not just real and tangible in the way that a stone was real and tangible, or even the way a child was. It was more than that. She was hyper-real. The _opposite _of void. To enter the space between the worlds was not in her nature. The Doctor could make her go, of course. He could make her go anywhere. He would have to, if he planned to penetrate the dark mystery at the heart of the universe.

But he couldn't bring her home.


End file.
